Process for the separation of acetylene and olefins



May 24, 1960 F. F. A. BRAcoNlER ET AL 2,937,714

PROCESS FOR THE SEPARATION OF ACETYLENE AND OLEF'INS Filed June 6, 1957 PROCESS FOR THE SEPARATION OF ACETYLENE AND OLEFIN S Frdric Franois Albert Braconier, Plainevaux, and Henri Adelin Hannaert, Liege, Belgium, assiguors to Societe Belge de lAzote et des Produits Chimiques du Marly, Liege, Belgium Filed June 6, 1957, Ser. No. 663,914

Claims priority, application Belgium Aug. 25, 1956 7 Claims. (Cl. 18S-115) This invention relates to processes for the treatment of gases from the pyrolysis of hydrocarbons, to separately extract acetylene and olenic hydrocarbons in the pure state.

According to the treated hydrocarbons (either gaseous or liquid, saturated or unsaturated) and the kind of the pyrolysis treatment (partial combustion, regenerators, electric arc, etc.) to which said hydrocarbons are subjected, the acetylene and olens contents of said pyrolysis gases vary in wide limits.

For extracting unsaturated hydrocarbons, including 'acetylene and olefins, from gaseous mixtures, it has already been proposed to treat said mixtures with liquid substances, which are normally gases or vapours at ordinary temperature and under atmospheric pressure, and to separate thereafter the unsaturated hydrocarbons. Ammonia in particular has been used at a temperature between -60 and 70 C. and under a pressure of some atmospheres, under which conditions the ammonia exlsible to completely separate acteylene and olefins while I reducing processing expenses.

Acetylene has been preferentially absorbed from a mixture with-olefins by washing the mixture with liquid ammonia advantageously under atmospheric pressure and at a temperature between about -60 and 70 C. According to the invention the process is carried ron by desorbing olefns together with a portion of acetylene from this solution and returning the so desorbed gaseous mixture to the absorption zone.

It has been observed that by washing a pyrolysis gas which contains, in addition to acetylene, variable and sometimes important quantities of oleiinic hydrocarbons, namely ethylene and propylene, with liquid ammonia under atmospheric pressure, the amount "of oleiins dissolved into the ammonia varies with the quantity of acetylene present. When the concentration of acetylene in the total gas being used increases, the absorption of olefins (in percent by volume of the total gas) by the liquid ammonia decreases and comes to a minimum (but in any case never to zero) and then increases-again.

Thus, it is not possible completely torseparate acetylene from olens by mere washing with liquid ammonia; and-it is therefore necessary to treat the ammoniacal solution of unsaturated hydrocarbons to separate therefrom acetylene and oleins in the pure state.

According to this invention separation is readily and economically yeffected by `subjecting the ammonical solu- Vtion tor increasedupressure and then to adegasiying by partial reduction of pressure. 'The olefins arel preferential- United States Patent() ly desorbed from the solution, which then contains only acetylene, the latter being thereafter separated by distillation.

Referring to the drawing, a schematic ow sheet type of representation is shown indicating the sequence, inter-relation, reactants, and resultantrs, of the various process steps of a process embodying and for practicing this invention in accordance with the legends appearing on the drawing and as disclosed herein.

The eiciency of the separation of olefins from the ammoniacal solution depends on the operating conditions in the desorption zone, namely the temperature and the pressure. This factor is particularly determinative as shown, for example, in the following table giving the results of tests effected at temperatures between'25 and.

40 C., and under a number of different pressures, on an ammoniacal solution containing 95.4% by volume of Hydrocarbon Retained in Ammonia After Degasiiying (percent by volume) Pressure Before Degasifying C2H2 Olens 12 atmospheres 98. 9 1.1 14 atmospheresf 99. 4 0.6 16 atmospheres. 99. 95 0.05 18 atmospheres. 99. 99 0. 01

According to these data, it results that by degasifying under a pressure of atleast l2 atmospheres and at a temperature between about 25 and 40 C., a substantially complete elimination of the olenic hydrocarbons from the ammoniacal solution is readily and economically obtained.

Proceeding according to this invention we return to the absorption zone said olenic hydrocarbons containing also a portion of actylene and usually some ammonia entrained from the ammonical solution during the Vdegasifying, and to mix them with fresh pyrolysis gas which is then subjected to washing with liquid ammonia.

-It has been observed, under the above rspecified working conditions for the degasifying, that the olens desorbed from the ammoniacal solution contain also such an amount of acteylene that, by recycling this gaseous mixture of olefins and acetylene to the absorption zone, the acetylene concentration of the feed gas is increased in the gaseous mixture actually treated with liquid ammonia. In this Way, the absorbing power of liquid ammonia for the olelins is reduced, said liquid ammonia dissolving acetylene preferentially and leaving a residual gas rich in oleiinic hydrocarbons which containsV substantially dinarily poor in acetylene, the concentration thereof may be,v increased not only by recycling the desorbed unsaturated hydrocarbons in the degasifying zone, but also by means of pure acetylene. It is however more advantageous to modify the working vconditions during the degasifying so as to desorb from the ammoniacal solution, a greater proportion of acetylene, which is recycled together with the olenic hydrocarbons. t f

Since the temperature of the absorption zone 60 to 70 C.) is obtained by vaporization of liquid ammonia, it is advantageous to remove, by washing with water, the gaseous ammonia entrained with the unsaturated hydrocarbons desorbed and recycled to the absorption zone. This is done, for example, by washing with waterorrother ammonia-absorbing material.

The process of this invention thus makes it possible to effect purification and concentration of acetylene diluted with olens, and to separately obtain said acetylene and olefinic hydrocarbons respectively in pure state. The following working conditions should be observed, for best results:

Absorbing the pyrolysis gas by liquid ammonia at 60 to 70 C., under atmospheric pressure with the formation of an acetylene and olens solution in said ammonia;

Desorbing the olefinic hydrocarbons and a portion of acetylene from said ammoniacal solution by partial degasifying at a temperature between 25 and 40 C., under a pressure of at least 12 atmospheres;

Recycling the desorbed gaseous mixture to the absorption zone, after having removed the entrained ammonia therefrom;

Separating acetylene from the ammoniacal solution produced in the desorption zone;

Eliminating the ammonia entrained with the residual gas from the absorption and concentrating the olens by known processes, e.g., liquefying and distilling.

The following is an illustrative example of one embodiment of the invention applied to the treatment of a light petrol pyrolysis gas:

Example 1 Percent Acetylene 10.9 Propylene 6.1 Ethylene 15.8

After absorption with liquid ammonia, the composition, by volume, of the dissolved gas mixture is as follows:

Percent Acetylene 95.4 Propylene 3.9 Ethylene 0.7

This ammoniacal solution, is raised to 18 atmospheres pressure, is degasied by a partial pressure reduction and then is separated by fractional evaporation to give substantially pure acetylene, the analysis of which shows:

Percent Acetylene 99.9 Propylene 0.1 Ethylene For the production of 1 ton pure acetylene, the quantities to be used are as follows:

The pyrolysis gas comprising 1.039 tons acetylene, 1.439 tons propylene and 2.638 tons ethylene, and the recycling gas comprising 0.665 ton acetylene, 0.091 ton propylene and 0.010 ton ethylene from the desorption zone are introduced into the absorption column. This gaseous mixture is treated with 15.7 tons liquid ammonia under atmospheric pressure and at a temperature of 70 C. 0.034 ton acetylene, 1.438 tons propylene and 2.638 tons ethylene are evolved from said column together with 2.2 tons entrained ammonia, which is removed by treatment with sulfuric acid.

The ammoniacal solution leaving the absorption column comprises 13.8 tons ammonia, and dissolved therein 1.670 tons acetylene, 0.092 ton propylene and 0.01 ton ethylene.

This efliuent solution is subjected to 18 atmospheres pressure and introduced at the lower third of a degasifying stripping column, the bottom of which is heated to a temperature of 42 C. At the top of this column, is recovered a gaseous mixture released therein by selective desorption as described above. The entrained ammonia is removed from this mixture by washing with water, and the residue of the mixture is then returned to the washing column.

A11 ammoniacal solution consisting of 12.918 tons ammonia, 1.003 tons acetylene Aand 0.001 ton propylene and free from ethylene is drawn off from the bottom of the degasifying column.

The fractional evaporation of this solution yields:

Acetylene which is washed with water for removing the entrained ammonia, giving 1 ton pure acetylene;

Ammonia, which is distilled and liquefied, making it possible to return 11.865 tons liquid ammonia to the absorption zone.

The process of this invention is suitable, mutatis mutandis, to any gas consisting of acetylene diluted with olefinic hydrocarbons and other gaseous components, namely to gases containing only one olen. When this olefin is, for example, ethylene, for which the absorbing power of liquid ammonia is particularly weak, the quantity of acetylene to be recycled from the desorption zone to the absorption zone may be reduced.

The following example gives the quantities to be used during the treatment of 1.025 tons acetylene and 2.500 tons ethylene. It shows also that the quantity of recycled acetylene represents only 8% of the whole quantity of acetylene treated in the absorption zone, as compared with 40% in the treatment of an acetylene, propylene and ethylene mixture in Example 1.

Example 2 Fresh pyrolysis gas containing 1.025 tons acetylene and 2.500 tons ethylene and a recycled gaseous mixture from the desorption column consisting of 0.078 ton acetylene and 0.024 ton ethylene are introduced into an absorption column. This gaseous mixture is washed with 14.3 tons liquid ammonia, including 11.460 tons recycled ammonia.

A gaseous mixture consisting of 2.499 tons ethylene evolves from this absorption column. The entrained ammonia (1.500 tons) is then removed from said mixture by treatment with sulfuric acid.

The ammoniacal solution (12.8 tons ammonia, 1.085 tons acetylene and 0.025 ton ethylene) drawn off from the desorption column,is subjected, under pressure of 18 atmospheres, to a partial desorption in the degasifying column at an average temperature of 3536 C. The desorbed gaseous mixture is washed with water for eliminating entrained ammonia (0.26 ton) and then returned to the absorption column. The ammoniacal solution of acetylene is treated as in Example 1 and gives -1 ton pure acetylene and 11.460 tons ammonia, which is also returned to the absorption column.

What is claimed is:

l. In a process for treatment of a gaseous mixture consisting essentially of acetylene and normally gaseous olens for obtaining substantially pure acetylene and olefin fractions using liquefied ammonia as an absorbing and washing solvent, the steps which comprise washing said gaseous mixture with liquefied ammonia at substantially atmospheric pressure for absorbing substantially all the acetylene from said mixture and forming a solution of acetylene and some olefins in said ammonia, increasing the pressure on said solution into the range of approximately 12 to 18 atmospheres, degasifying said solution with reduction of pressure of about 2 atmospheres for removing therefrom a gaseous phase comprising a major portion of the dissolved olefins with a small portion of acetylene and ammonia 'vapor and leaving a residual solution comprising acetylene dissolved in ammonia with substantially no olefin, and separating said acetylene from said residual ammonia solution.

2. A process as in claim 1 in which said acetylene is separated from said ammonia solution by distillation of said solution.

3. A process as in claim 1 in which the gaseous phase of said degasifying step is returned and recycled to said absorption and washing step for mixture with said gaseous mixture therein for increasing the concentration of acetylene in said ammonia solution produced in said step and for reducing the amount of said olens absorbed in said absorption and washing step.

4. In a process for treatment of a gaseous mixture consisting essentially of acetylene and normally gaseous olelins for obtaining substantially pure acetylene and olefin fractions using liquefied ammonia as an absorbing and washing solvent, the steps which comprise washing said gaseous mixture with liquefied ammonia at a temperature less than the boiling point of ammonia at atmospheric pressure for absorbing substantially all said acetylene from said mixture in said liqueed ammonia along with some of said olens forming an ammonia solution of said absorbed gases, increasing the pressure on said resulting ammonia solution into the range of about 12 to 18 atmospheres, degasifying said ammonia solution with expansion and pressure reduction of about 2 atmospheres for removing from said solution a gaseous phase comprising a major part of said dissolved olefins and a small portion of acetylene and resulting in a residual solution comprising essentially pure acetylene dissolved in ammonia with substantially no oleiins, and separating said acetylene from said ammonia.

5. A process as in claim 4 in which the gaseous phase of said degasifying step is recycled through said washing and absorbing step and mixed therein with said gaseous mixture for increasing the concentration of acetylene in said ammonia solution.

6. In a process for treatment of a gaseous mixture consisting essentially of acetylene and normally gaseous oleiins for obtaining substantially pure acetylene and olen fractions using liquefied ammonia as an absorbing and washing solvent, the steps which comprise washing said gaseous mixture with liquefied ammonia for absorption from said mixture of substantially all the acetylene there- -in and forming a liquid ammonia solution of said acetylene with some of said olefin, subjecting said resulting solution to a pressure increase of at least about l1 atmospheres, degasifying said pressurized solution with a pressure reduction of about 2 atmospheres for separating therefrom a gaseous phase comprising a major p0rtion of the dissolved olefin and a small portion of dissolved acetylene and leaving a residual solution comprising substantially pure acetylene dissolved in ammonia with substantially no dissolved olens, and separating said dissolved acetylene from said ammonia solvent by distillation of said residual ammonia solution.

7. A process as in claim 6 in which said gaseous phase removed during said degasifying step is recycled to said Washing and absorbing step and admixed therein with said gaseous mixture for increasing the concentration of acetylene -in said absorbing step and decreasing the amount of oletins absorbed in said washing and absorbing step.

References Cited in the le of this patent UNITED STATES PATENTS 2,029,120 Schilling et al. Jan. 28, 1936 2,250,925 Babcock July 29, 1941 2,714,940 Milligan Aug. 9, 1955 2,812,829 Marullo et al. Nov. l2, 1957 

1. IN A PROCESS FOR TREATMENT OF A GASEOUS MIXTURE CONSISTING ESSENTIALLY OF ACETYLENE AND NORMALLY GASEOUS 